1. Field of the Invention
The present invention relates to a power supply controller used for a device control circuit of electric equipment in a mechanical apparatus such as a car.
2. Description of the Related Art
A power supply controller supplies a drive voltage to a control circuit for electric devices mounted on a mechanical apparatus such as a car by generating a specific operating voltage such as 5 V or 3.3 V from a battery voltage. However, there is an increasing request from customers for ensuring operations against a battery voltage drop when an engine starts. To satisfy this demand, there is a need for a power supply controller featuring both an up conversion function and a down conversion function.
FIG. 18 shows a conventional example of the power supply controller. In FIG. 18, a battery 1 is connected to a smoothing circuit 4. A pulse width modulation (PWM) control section 9 includes a down conversion control circuit 9a and an up conversion control circuit 9b. The down conversion control circuit 9a drives a switching device for down conversion 3. The up conversion control circuit 9b drives a switching device for up conversion 5. The battery generates a primary voltage 6 using the switching device for down conversion 3 and the switching device for up conversion 5. The smoothing circuit 4 includes diodes 4a and 4c, an inductor 4b, and a capacitor 4d. 
In order to decrease an effect of battery voltage variation, the PWM control section 9 allows a reference voltage generation circuit 10 to generate a reference voltage 10a from the battery voltage 1a or the primary voltage 6 whichever is higher. The battery voltage 1a passes through a voltage divider 18 and is supplied to a voltage comparator for battery voltage monitoring 14. The voltage comparator for battery voltage monitoring 14 compares the battery voltage 1a with an up conversion switching voltage 16a and a down conversion switching voltage 16b (16a<16b) predetermined in an up/down conversion switching voltage setting circuit 16 for determination. The voltage comparator for battery voltage monitoring 14 outputs battery voltage monitoring information 14a after the determination. An up/down conversion switching circuit 15 processes the battery voltage monitoring information 14a and notifies up/down conversion switching information 15a to the down conversion control circuit 9a and the up conversion control circuit 9b in the PWM control section 9.
The primary voltage 6 passes through the voltage divider 8 and is supplied to the voltage comparator for primary voltage monitoring 7. The voltage comparator for primary voltage monitoring 7 compares the primary voltage 6 with the reference voltage to detect a level of the primary voltage 6. An output 7a is fed back to the PWM control section 9 for generating the primary voltage that is modified by controlling the switching device for down conversion 3 or the switching device for up conversion 5.
FIG. 19 shows operational waveforms of the signals in FIG. 18 when the power is turned on. Normally, the battery voltage 1a increases when the battery 1 is connected to the power supply controller at timing 300. The reference voltage 10a generated from the battery voltage 1a also increases as indicated by reference numeral 304. When the reference voltage 10a reaches a predetermined value, the up/down conversion switching information 15a outputs 0 (down conversion control notification). The down conversion control circuit 9a starts stepping down the primary voltage 6.
The voltage comparator for battery voltage monitoring 14 monitors a voltage based on the battery voltage 1a and the reference voltage 10a and may incorrectly determine detection of the up conversion switching voltage 16a while the battery voltage 1a or the reference voltage 10a is in a increasing process as indicated by reference numeral 301. At this time, the up/down conversion switching information 15a outputs 1 (up conversion control notification). The up conversion control circuit 9b of the PWM control section 9 operates and supplies a large current to the battery smoothing circuit 4 to increase the primary voltage 6.
A large current may occur while the battery voltage 1a is in a increasing process and the reference voltage generation circuit 10 is in a generating process of the reference voltage 10a. In such case, the voltage decreases due to the impedance of a battery connection cable, causing a local decrease in the reference voltage 10a as indicated by reference numeral 305. The decrease of the battery voltage 1a causes the voltage comparator for battery voltage monitoring 14 to incorrectly determine detection of the down conversion switching voltage 16b. The up/down conversion switching information 15a outputs 0 (down conversion control notification) to stop the up conversion control circuit 9b of the PWM control section 9 and operate the down conversion control circuit 9a. The down conversion control decreases the current to the battery 1, stops decreasing the voltage due to the cable, and increases the battery voltage 1a. 
The voltage comparator for battery voltage monitoring 14 may become incapable of correctly monitoring the battery-voltage 1a while the battery voltage 1a is in a increasing process as indicated by reference numeral 302 and the reference voltage generation circuit 10 is in a generating process of the reference voltage 10a as indicated by reference numeral 304. As a result, the up conversion control circuit 9b and the down conversion control circuit 9a repeatedly switch between down and up conversion operations based on the incorrect determination to increase and decrease a current from the battery 1 and ripple the battery voltage 1a. The up/down conversion switching information 15a causes an unstable up/down conversion switching state as indicated by reference numeral 307 while the battery voltage 1a is in the increasing process as indicated by reference numeral 302. The switching device for up conversion 5 erratically operates to abnormally increase the primary voltage 6 as indicated by reference numeral 308.
FIG. 20 shows operational waveforms corresponding to the following. The battery voltage 1a steps down to the up conversion switching voltage 16a specified by the up/down conversion switching voltage setting circuit 16. The PWM control section changes from the down conversion operation to the up conversion operation to recover the battery voltage la to the down conversion switching voltage 16b or higher.
At timing 401, the battery voltage 1a is lower than the up conversion switching voltage 16a. The voltage comparator for battery voltage monitoring 14 causes output of the up/down conversion switching information 15a set to 1 (up conversion control notification). The up conversion control circuit 9b operates to generate the primary voltage 6. The up conversion control circuit 9b in the PWM control section 9 controls the switching device for up conversion 5 to stepwise increase a current from the battery 1. The battery voltage 1a ripples due to an impedance of a connected harness.
During the ripple under the up conversion control, the battery voltage 1a recovers and reaches the down conversion switching voltage 16b at a time point 403. The up/down conversion switching information 15a of the up/down conversion switching circuit 15 outputs 0 (down conversion control notification). However, the battery voltage 1a still ripples under control of the up conversion control circuit 9b and becomes lower than the up conversion switching voltage 16a again. The up/down conversion switching information 15a outputs 1 (up conversion control notification).
A specified hysteresis voltage width is provided between the up conversion switching voltage 16a and the down conversion switching voltage 16b. However, the rippling battery voltage 1a makes an up/down conversion switching operation unstable near the down conversion switching voltage 16b or the up conversion switching voltage 16a as indicated by timing 403. The up conversion control circuit 9b in the PWM control section 9 does not generate a normal control pulse. The erratically generated up/down conversion switching information 15a passes control to the switching device for up conversion 5. The primary voltage 6 abnormally rises as indicated by reference numeral 405.
FIG. 21 shows operational waveforms when the battery is removed from the power supply controller. The battery voltage 1a decreases when the battery 1 is removed at timing 501. The battery voltage 1a is lower than the up conversion switching voltage 16a at timing 502. The voltage comparator for battery voltage monitoring 14 causes output of the up/down conversion switching information 15a set to be 1 (up conversion control notification). The up conversion control circuit 9b operates to generate the primary voltage 6.
When the battery voltage 1a further decreases to point 503, the decrease exceeds a limit that can be controlled by the up conversion control circuit 9b in the PWM control section 9. Accordingly, the primary voltage 6 decreases. The reference voltage 10a also decreases. The voltage comparator for battery voltage monitoring 14 or the voltage comparator for primary voltage monitoring 7 cannot correctly monitor voltage. Since the battery voltage 1a ripples due to the up conversion control, the reference voltage 10a becomes unstable as indicated by reference numeral 504. Consequently, the up/down conversion switching information 15a also becomes unstable as indicated by reference numeral 505. Control is passed to the switching device for up conversion 5 at an incorrect timing. The primary voltage 6 abnormally rises as indicated by reference numeral 506.
As a conventional solution, Patent Document 1 discloses a method of stabilizing linear voltage characteristics by supplying a linear regulator circuit and a switching regulator circuit with a stable reference voltage generated from another system.
Patent Document: JP-A No. 168043/1994
Recently, there is an increasing demand for ensuring operations at a low battery voltage. A power supply controller is requested to ensure operations of circuits using a battery even when a battery voltage drops due to a cranking noise generated during starter activation. Specifically, operations are requested to be ensured even when the battery voltage becomes lower than a primary voltage generated by a switching regulator. According to a conventional technology, voltage monitoring circuits cannot monitor the battery voltage while the battery voltage is increasing. An incorrect up/down conversion switching operation is repeated to abnormally increase the primary voltage 6.